Modeling a terrain accurately from a contour map
Assuming your landscape is a mesh, you may try the following:


add a plane a little bigger than your landscape, and at the base (or lower than) of landscape
with plane selected, and under "modifier tab", you add an "array" modifier. You set to constant offset, X and Y to 0, Z to a distance that suits your needs, and you should increase the count to get as much planes as you need to cover the top (or pass a bit) of your landscape). If your landscape is in "real" scale, you can set the Z to 10m, to get the 10m isocontours.
with plane selected, under "boolean" you set to "intersect" with the landscape
apply your modifiers to the plane (you will have horizontal planes by choosen iso Z value)
press "space", write "convert to" and select "curve from mesh/text" (you will convert the planes to lines, to get their perimeter - if you need, you can then increase thickness of lines, etc)
optional step if you want to control the material (colours, etc) of your contours: you  again "convert to" "mesh from curve/text"


Following the exact steps mentioned above you get the following result: